Slurries of oxide nanoparticles have widespread applications as polishing and lapping agents for photonics and for chemical mechanical planarization (CMP) of microelectronic devices. The presence of aggregates or other oversize contaminants in CMP slurries is highly undesirable because of their adverse effect on surface quality, even at part-per-billion concentrations. Efforts to prevent aggregation and eliminate contaminants are hampered by a lack of techniques for detecting and characterizing comparatively small numbers of large particles in an ocean of nanoparticles. Direct imaging, laser occultation and light-scattering techniques, for example, are ruled out by the slurries' turbidity and by the lack of contrast between the nanoparticles and the larger contaminants. Conventional particle counters are clogged and fouled by slurry particles at full concentration. Remedying these problems by dilution is impractical both because of the very large volume of fluid that then would have to be analyzed and also because the process of dilution can influence aggregation processes that might be of interest.